[Therion] Breaking a loop at a specific station on extended elevation

[Tarquin Wilton-Jones]

> Pretty sure this is the way it is.  Which is why I tend to use extend <leg> rather than with a single <station>.

Yes, I think this might be something I will end up relying on, because
it is much more readable, and understandable. And on top of that, you
know that even if Therion gets changed to follow Survex, at least your
Therion data will work either way.

> Extend <station> in Therion is the same as the way PocketTopo implements 'flip' (equivalent to Therion's 'extend').

Interesting. So we have three existing implementations, but two
different mechanisms (Survex does it the other way). Most of Therion's
data syntax is compatible (and directly interchangeable) with Survex.
This is an unusual case where it isn't compatible.